1. Field of the Invention
The invention relates generally to the field of well logging instruments. More specifically, the invention relates to devices used to position well logging instruments as precisely as possible in the center of a conduit or casing disposed in a subsurface wellbore, or in the center of the wellbore itself when no casing is used.
2. Background Art
Well logging instruments known in the art include an instrument used to provide services under the mark USI-ULTRASONIC IMAGER TOOL, which is a mark commonly owned with the assignee of the present invention. The foregoing instrument, among others, is inserted into a pipe or casing cemented in place in a wellbore drilled through subsurface rock formations. Information obtained from the instrument is used to evaluate the quality of the cement disposed in an annular space between the exterior of the casing and the wall of the wellbore. As is known in the art, the cement is intended to hydraulically isolate the formations outside the casing from each other and to externally seal the casing in the wellbore.
Instruments such as the foregoing USI instrument emit pulses of acoustic energy, typically at frequencies of 100 KHz and above, and detect reflected acoustic energy. The transmitting acoustic pulses and receiving reflected energy may be performed using a single transducer disposed on a device which rotates the transducer about the longitudinal axis of the instrument. Condition of the cement may be inferred by the amplitude and wave characteristics of the detected acoustic energy, and because such emission and detection is performed using a rotating transducer, the evaluation may be circumferentially differentiated.
Accurate evaluation of the cement condition using acoustic devices such as the USI instrument described above, however, requires that the instrument is disposed as closely as possible in the center of the casing. In the case of the foregoing USI instrument, having the instrument be disposed more than about 0.15 inches from the center of the casing results in lower quality of ultrasonic transit time data that generates the basis for acoustic impedance curves and color graphics for the cement map generated from the reflected acoustic signals. The signals can become essentially uninterpretable when the tool eccentering exceeds about 0.30 inches. As a general rule (not that this rule is actually dependent on casing O.D. and casing weight) the maximum eccentering that can be tolerated with the USI instrument may be defined by following expression:Eccentering Limit (inches)=(0.1)*(thickness)*(Casing O.D.)
The USI instrument, as is the case for many other cement evaluation instruments, uses bowsprings to urge the instrument into the center of the wellbore casing. The effectiveness of such bowsprings depends on, among other factors, the spring rate, the number of springs, the weight of the instrument and the inclination of the wellbore from vertical. Beyond a certain point, it is impracticable to increase the spring rate of bowsprings or their number with respect to any particular size and weight well logging instrument.
It is desirable to have a device to increase the effectiveness of bowspring centralizers without the need to increase the spring rate of the bowsprings, the number of bowsprings or the size of the bowsprings.